The present invention relates to crosslinked gelling agents employed in treatment fluids during subterranean operations, and more particularly, to the use of oligomeric and polymeric electronically-modified boronic acids to provide crosslinked gelling agents that are stable at high operating temperatures while allowing reduced gelling agent loadings.
In treatment fluids that use boron-based reagents to produce crosslinked gelling agents, such as crosslinked guar, there is typically interplay between the nature of boron crosslinking, the pH, and temperature. This relationship is indicated, for example, in FIG. 1, for the prototypical boron crosslinker boric acid. The chemical reaction between crosslinked boron and non-crosslinked boron is considered to be reversible with a fairly low barrier to crosslinking and de-crosslinking. At room temperature and at reasonably high pH (around 8.5) crosslinking is favored. As temperature increases, the barrier of activation energy between the crosslinked and non-crosslinked material becomes insignificant relative to the energy of the system, and the gel de-crosslinks to form linear polymers that may or may not have boron bound intramolecularly. This can be demonstrated experimentally, as shown in FIG. 2, by plotting viscosity as a function of temperature. Notably, when this exemplary boronic acid-acrylamide-based gel de-crosslinks in run 1 the temperature of de-crosslinking is 180° F. Subjecting the same material to the same temperature ramp a second time (run 2) causes the gel to de-crosslink at a lower temperature 160° F., indicating possible irreversible chemical alteration of the polymer system during run 1.
These linear polymers are not desirable in fracturing operations where the use of such crosslinked gels in treatment fluids is common. Hydraulic fracturing techniques are widely used to enhance oil and gas production from subterranean formations. During hydraulic fracturing, a fluid is injected into a well bore under high pressure. Once the natural reservoir fracture gradient is exceeded, the fracturing fluid initiates a fracture in the formation that generally continues to grow during pumping. The operation generally requires the fluid to reach a maximum viscosity as it enters the fracture affecting both the fracture length and width. Among the issues that arise with linear polymers is that they do not have the necessary viscosity for proper proppant transport at elevated temperature.
While boron-based crosslinking agents may be effective for many types of fracturing fluids, a certain amount of the gelling agent is needed to achieve the viscosity necessary to fracture the formation and support transport of the proppant. However, it is generally desirable to use as little gelling agent as possible in a fracturing fluid so that the overall cost of the fracturing job is lower and less polymer residue remains in the fracture and the proppant pack after breaking down the crosslinked gel. In this regard, use of less gelling agent can help minimize formation damage.
Recent advances in reducing the amounts of gelling agents include the use of boronate-functionalized polymers that may exhibit similar energies of activation (Ea) as the boric acid prototype. However, boronate-functionalized polymers typically dissociate into independent linear polymers at lower temperatures. For example, a typical borate gel may be held stable for several hours at 250° F. and a pH of about 11, while boronate-functionalized polymers may dissociate into linear polymers at temperatures of only about 180° F., well below a desirable operational temperature for certain applications.
Other issues that may arise with boron-based crosslinking systems relates to compatibility with calcium ion. There has been an increasing demand to use CaCl2 brines in offshore operations. This is due, at least in part, to the fact that CaCl2 brines are less expensive than other brines. However, current boron crosslinking processes are not compatible with such brines. In particular, the elevated pH employed in boron crosslinking can cause calcium precipitation.